Our invention relates to the preparation of filled thermoplastics. In particular, our invention relates to the graft polymerization of thermoplastic polymers onto and within the interstices of various rigid fillers to give filler-plastic combinations which, because of the chemical bonding between the filler and the thermoplastic, can be processed by conventional methods, e.g., extrusion.
The incorporation of rigid fillers such as glass, talc, silica, wood flour, limestone, asbestos, carbon black, zinc oxide, or alumina into thermoplastics has been practiced for years, and techniques for preparing filled thermoplastics are well known in the prior art [R. Seymour, Chem. Technol. 4: 422 (1974)]. Fillers are added to thermoplastics as extenders to give a less expensive final product and, more recently, to conserve valuable and dwindling petrochemical-based resources used in plastics production. Fillers are also added under certain circumstances to alter the physical properties of a manufactured plastic article, e.g., increase elastic modulus and reduce creep [L. E. Nielsen, Trans. Soc. Rheology 13(1): 141 (1969)].
Fillers generally become more compatible with the matrix polymer in which they are imbedded, if a small amount (i.e., less than 5%) of polymer is graft polymerized onto the filler surface. This small amount of grafted polymer need not be chemically the same as the thermoplastic matrix. The use of these graft polymerized fillers also produces plastic composites with high tensile strengths. The preparation of these graft polymerized fillers and their incorporation into plastics is known in the prior art [H. G. G. Dekking, J. Appl. Polym. Sci. 9: 1641 (1965) and I. E. Neimark, A. A. Chuiko, I. B. Slinyakova, SPE Trans. 2: 135 (1962)].
Fillers, including those containing a small amount of graft polymer, are generally blended with the thermoplastic matrix by mechanical mixing in amounts which may be as high as 50 volume percent of the total mixture. Mixing may be carried out on mill rolls or in a Banbury mixer and is done prior to extrusion processing. This separate mixing step is inconvenient and time consuming and thus adds to the cost of the finished plastic product. Moreover, completely homogeneous blends of the two basicly incompatible components are difficult to achieve.
We have discovered that rigid fillers, which have been graft polymerized with a sufficiently high percentage of a thermoplastic polymer (ideally, 40-50%, by weight) may be processed directly, in the absence of any other polymer or additive, into useful plastic articles by conventional methods, such as extrusion. Unlike the lightly grafted fillers in the prior art, the synthetic polymer portion is located not only on the filler surface but within the filler matrix. Thus, in the products prepared by the instant invention, the thermoplastic matrix polymer, which normally suspends the filler and which is a characteristic component of all conventional filled composites, has been completely eliminated to give a new plastic composition composed entirely of grafted rigid filler. The new composite compositions have a major advantage over the thermoplastic matrix-filler combinations known in the prior art, since they are a single chemical species (graft copolymer) and are thus completely homogeneous at the outset. A separate and costly mixing step prior to extrusion is therefore not required. Further, by the method of manufacture of this unique "filler-matrix" composite system the filler cannot aggregate but is uniformly and permanently distributed throughout the composite system.